1. Field of the Invention
The present invention relates to a capacitor, and more particularly, to a storage capacitor for a liquid crystal display (LCD) device and method for manufacturing the same.
2. Discussion of the Related Art
In general, there are two kinds of LCD devices. One type uses active driving elements and the other type uses passive driving elements. An LCD device which includes a substrate having thin film transistors (TFTs) in an array of rows and columns as active switching elements is called an active matrix liquid crystal display (hereinafter "AMLCD") device. In AMLCD devices, two substrates are joined which face one another. One substrate has multiple TFTs in a matrix arrangement and the other substrate has a color filter. A liquid crystal material is injected between the two substrates.
FIG. 1 is a plan view of the active matrix substrates having the TFTs and FIG. 2 is a cross-sectional view of FIG. 1 taken along line I-I'
Referring to FIGS. 1 and 2, an active matrix substrate for a liquid crystal display device of the present invention includes a plurality of gate bus lines 2 and a plurality of source bus lines 3 that are formed such that they cross each other over an insulating substrate 1. A pixel area is formed surrounded by the gate bus line 2 and the source bus line 3. A TFT 4 is located at an intersection of the gate and source bus lines.
Referring to FIG. 2, a gate electrode 12 and a gate bus line 2 are formed on a substrate 1 and then anodized to form an oxide layer 11. A gate insulating layer 15 is formed over the gate electrode 12, gate bus line 2, and the substrate 1. A semiconductor layer 16 is formed over the gate insulating layer 15 and an ohmic contact layer 17 is formed over the semiconductor layer 16. Source and drain electrodes 13 and 5 are formed over the ohmic contact layer 17. A pixel electrode 10 is then formed to contact the drain electrode 5 and over the gate insulating layer 15.
A light shielding film as wide as the TFT is formed in a mesh structure to protect the TFT from being exposed to light. Then, the aperture of the light shielding film is aligned with the pixel area. Thus, the TFT 4, the gate bus line 2, and the source bus line 3 are covered with the light shielding film. The light shielding film can be formed on any side of the two substrates. In the TFT 4, a gate electrode 12 is connected to the gate bus line 2, and a source electrode 13 is connected to the source bus line 3. The pixel electrode 10 is formed at the pixel area and as wide as the pixel area. The drain electrode 5 is connected to the pixel electrode 10. A common electrode 22 (FIG. 3) is formed on the substrate having the color filter, which faces the active matrix substrate. A liquid crystal capacitor C is formed between the common electrode 22 and the pixel electrode 10.
When the TFT 4 is turned ON by the signal supplied to the gate electrode 12 through the gate bus line 2, the data signal supplied to the source electrode 13 through the source bus line 3 is induced to the pixel electrode 10. In such a case, the liquid crystal is driven by the conducted charge at the liquid crystal capacitor. When the TFT is turned OFF, the conducted charge at the liquid crystal capacitor remains until the next signal through the gate bus line 2 is supplied again to the gate electrode 12. However, in that case, the source electrode 13 and the drain electrode 5 are not perfectly insulated and becomes similar in condition to a high resistance material even though the TFT 4 is OFF. Accordingly, the conducted charge slowly leaks out from the liquid crystal capacitor as time passes. In order to prevent this problem, refreshing cycles are needed for recharging of the liquid crystal capacitor at specific time intervals. In addition, the capacitor C has to be larger in order to reduce the number of refreshing cycles. For such reasons, a storage capacitor Cs is formed in parallel with the liquid crystal capacitor C to help keep the amount of the conducted charge of the pixel electrode 10. FIG. 3 is an equivalent circuit diagram of the TFT 4, the liquid crystal capacitor C, and the storage capacitor Cs.
For the AMLCD manufactured by using a conventional method as mentioned above, in order to maintain a bright contrast with a small power consumption, the ratio of aperture, i.e., the ratio of the area of the pixel electrode 10 compared to the area covered by the light shielding film, should be increased as much as possible. To increase the ratio of the pixel aperture, the pixel electrode 10 should be enlarged while the area covered by the light shielding film should be reduced as small as possible.
When the light shielding film is reduced, the size of the TFT 4 should also be reduced as well as the width of the gate bus line 2 and the width of the source bus line 3. In doing so, the size of the storage capacitor 23 is reduced accordingly and the conducted charge is not sufficiently maintained to supplement the charge leakage. Therefore, in order to make the active matrix elements as small as possible while insuring a desired capacitance of the storage capacitor, a thin film may be used for the insulating layer, a material having a large dielectric value may be used, or the capacitance area of the storage capacitor may be enlarged by employing a stacked layer structure.
However, the conventional methods have the following problems. First, because the distance between the pixel electrode 10 and the gate bus line 2 is short, the charge leakage may increase or an overheating may occur when the insulating layer is formed with a thin film. Second, when a material having a large dielectric value is used, the quality of such material is worse than SiO or SiN. Third, when a stacked layer is used as the structure of the storage capacitor, a line-open occurs frequently because the process and the step-coverage become more complicated.